Various labels have been attached to articles so that the articles can be distinguished one from the other. For example, bar code labels are attached to articles of grocery and are scanned at a check-out counter in order to automatically identify the articles and to register the price of the articles as they are purchased.
Bar code labels have also been used in inventory control and monitoring. Accordingly, these bar codes may be scanned in order to track articles as they move into, through, and out of a storage area. It is also known to read the bar codes attached to articles in order to access various computer records regarding the articles.
Bar code labels, however, have several drawbacks. For example, computer stored records that are accessed when a bar code is read do not move with the corresponding article. Therefore, if the article to which the bar code label is attached is remote from the computer, the records concerning that article cannot be immediately accessed if necessary.
Moreover, bar code labels cannot be read remotely. Thus, if it is desired to take an inventory of articles currently in the storage area, personnel must physically scan each label on each article one at a time in order to determine which articles are presently in the storage area. Such scanning requires the physical presence of the personnel at the location of the articles and is extremely time consuming. Additionally, because bar code labels cannot be read remotely, they cannot be used as security devices that can be detected if the articles to which they are attached are improperly removed from a secured area.
Instead of bar coded labels, it is known to attach radio frequency identification (RFID) tags to the articles to be monitored. As in the case of bar code labels, the RFID tags contain unique identification codes so that the articles to which they are attached can be distinguished from one another. However, unlike bar code labels, reading RFID tags does not require the physical presence of personnel because the RFID tags can instead be read remotely. Thus, inventory can be taken more quickly because personnel are not required to walk around a storage area or other area in order to read the RFID tags. Moreover, because RFID tags can be read remotely, they can be used as security devices. Thus, if someone attempts to surreptitiously remove an article to which an RFID tag is attached from a secured area, a remote reader can sense the RFID tag and provide an appropriate alarm. Furthermore, it is also possible to provide an RFID tag with memory. Therefore, any records concerning the article to which the RFID tag is attached can be maintained on the RFID tag rather than in a remotely located computer.
These advantages of RFID tags make their use quite attractive. However, care must be exercised in using such RFID tags because the interrogation of RFID tags for their identification codes so that their corresponding articles can be identified can consume a substantial amount of transmission time and processing power.
Generally, two techniques have been used to read the identification codes of RFID tags. In one technique, any RFID tags within the transmission range of the tag reader respond at random to an interrogation from the reader. When there are a large number of RFID tags in the receiving vicinity of the tag reader, contention may be high so that repeated interrogations may be required to read all RFID tags. Therefore, this technique works well only when there is a small number of RFID tags to be read, and is very inefficient when there is a large number of RFID tags to be read.
In the other technique, the RFID tags respond to an interrogation in a predetermined fashion using a static selection criterion to avoid contention. This approach is more effective for systems having a large number of RFID tags, but requires the transmission of substantial amounts of data from the RFID tags to the tag reader.
These problems are exacerbated because the identification codes used on RFID tags are usually quite long. For example, long identification codes are frequently required where a great many RFID tags may be used in buildings or storage areas that are close enough in proximity for a tag reader in one building or storage area to inadvertently read the RFID tags in a nearby building or storage area. In this example, the identification codes must of necessity be quite long so that the RFID tags in the proximate buildings or storage areas are uniquely identified. As another example, the number of articles stored in a single location may be sufficiently large to warrant a long identification code. Because long identification codes are generally required, the amount of time required for the RFID tags to transmit their identification codes to the tag reader, and the amount of time required for the tag reader to process these identification codes upon reception are commensurately large.
The present invention overcomes one or more of these or other problems.